The present invention relates to a sealing apparatus for a pressurized borehole drilling pipe, and, more particularly, to a side entry clamp and packoff for permitting the sealed and secured side entry of a communication wireline from the outside of a drill pipe section therein.
Sealing apparatus for borehole drilling operations include structures referred to as "packoff" units for sealingly engaging elements in communication with pressurized members. Such packoffs are often used in drilling apparatus where drilling mud is pumped under pressure down a drill stem to a mud motor which drives a drill bit. The mud motor drilling operation is particularly advantageous for a type of borehole construction referred to as controlled drilling wherein the drilling head is directionally controlled to drill both vertically and laterally. Similarly, various forms of packoffs have been found effective in facilitating certain aspects of controlled drilling.
Controlled drilling by its very definition generally requires directional survey information, since, in order to assure that the intended hole controlled course and destination are achieved, it is necessary to know in which direction the hole is tending at any time. The bottom hole position with respect to the top hole position can be calculated from inclination and direction readings taken from a survey tool positioned in the borehole. It is the implementation of the survey tool that frequently requires the assembly of a packoff unit in the borehole apparatus.
Survey tools have been found to be useful for all forms of drilling, whether controlled or uncontrolled, since each form of drilling is greatly affected by forces which operate upon the drilling head and tend to randomly direct the course of the borehole away from that desired. Variations and hardness of formations, in particular, may cause the course to wander since the drill bit seeks the path of least resistance. Borehole courses are also affected by their reactive torque produced by a rotating drill bit, which operates upon the length of drill pipe and tends to produce a spiral hole. These forces continually affect the drilling operation and may cause deviations from the intended course of the borehole even when the drilling is theoretically controlled. It has thus been observed that surveying of the borehole in increments during which the drilling is momentarily stopped and a survey package is lowered into the hole, is oftentimes inadequate for desired controlled drilling accuracy. Techniques have thus been developed for continuous monitoring of borehole construction facilitating true "control" in drilling.
In the past drilling has been controlled either through "conventional" or "steering" procedures. Conventional directional drilling is generally a blind process in which a hole section is begun in a particular attitude and drilled for a time thereafter without knowledge of its instantaneous attitude. Steering, on the other hand, is a continuous process in which the attitude of the hole is continuously measured in order that course corrections may be made continuously rather than after a section of the hole has been drilled. Steering is most often a technique that is used in connection with the aforesaid mud motor drilling systems, in which the drill pipe remains stationary and the drill is rotated by a mud turbine, or moyno motor, operated by high pressure drilling mud supplied from the surface. Actual steering, or deflecting capability, is provided by a deflection tool known as a "bent sub", which is a substitute section of drill pipe formed with a bend therein and positioned at the lower end of the drill string near the drill bit. The primary determinate of the angle at which the drill bit addresses the formation is the degree of bend in the bent sub. In order to turn the hole the entire drill string is merely rotated at the surface to point the bent sub in a different lateral direction.
There are four general types of attitude indicating, or steering tool, instruments adapted for positioning in the borehole to provide directional information. These instruments include the gyroscopic, inertial, magnetic and gravitational types. Gyroscopic devices utilize the direction maintaining characteristic of a rotating body, while inertial devices, such as accelerometers, sense changes in direction by the principle of inertia. Magnetic devices generally use either a magnetic compass or flux gate compass to sense the earth's magnetic field. Gravitational devices characteristically use a pendulum to sense the earth's gravity field. The position of the attitude indicating devices is detected either photographically, mechanically, electrically or magnetically. The key to steering is communicating this information to the surface, wherein the utilization of a packoff generally becomes necessary.
Attitude sensing devices operate generally in either a drilling "interrupt" or a "while" drilling mode. Generally speaking, only the devices employing electrical or magnetic sensing elements can be used in the "while" drilling mode because of the necessity of transmitting the data up the drill string to the surface. In some instances, the actual transmission from the steering tool is via acoustical transducers which alleviate cumbersome wirelines. However, in most instances where assured reliability and cost are factors for the surface to the downhole communication link, a "wireline" is preferable. Such wirelines must interconnect steering tools inside the pressurized drill stem with instruments on the surface and outside the drill stem, necessitating a packoff at some point therebetween.
A conventional and commonly utilized prior art approach to steering through attitude sensing in the "while" drilling mode includes the communication wireline, wherein a cable is threaded through a packoff unit at the surface of the borehole near the end of the drill pipe and suspended through the center thereof. The wireline, in this manner, connects the steering tool and monitoring instruments at the surface. This approach, while reliable and effective in steering the drill bit to facilitate true "controlled" drilling, creates other serious procedural and mechanical problems which are directly related to the drilling operation. For example, a cable extending through the center of the drill pipe serves to complicate the requisite drilling hardware and the procedural aspect of adding additional sections of pipe which is inherently necessary as the borehole becomes deeper. Since the cable must be fed into the borehole through the drill stem from a cable reel, or drawworks, on the surface, that end of the wireline is not readily detachable. The only feasible way found to add additional drill pipe sections has been by pulling the steering tool and downhole end of the wireline out of the drill pipe, threading it through the newest section of pipe, and dropping the steering tool and wireline back into the hole. This procedure requires a steering tool which may be readily locked and unlocked in position in the drill pipe. There is also the problem of sealing the area of interconnection where the wireline enters into the drill pipe at the surface of the borehole. The end of the drill pipe must be in sealed communication with the mud pump which forces mud into the hole under pressure for driving the mud motor and carrying off cuttings. Therefore, the wireline must enter the drill pipe through the aforesaid surface "packoff" which is expensive and further complicates the procedure of adding additional drill pipe.
The apparatus of the present invention is especially adapted for drilling with a steering system by providing for a wireline to be suspended along the outside of the drill stem and to enter and be secured therein through a side entry packoff and clamp near the drilling head. In this manner the present apparatus overcomes many of the disadvantages of the prior art by providing a wireline communication link which enters the borehole through the upwardly moving mudflow which is egressing around the drill stem rather than a surface packoff. It may be seen that the surface packoff is effectively replaced by a "downhole", side entry clamp and packoff, and, the step of pulling the wireline and steering tool out of the hole to add additional drill stem may be eliminated. Since the outside wireline may still serve to connect the steering tool with the surface monitoring equipment, the method and apparatus of the present invention permits an effective "while" drilling steering mode without the major operational disadvantages generally associated therewith.